The invention is related to a method for the formation of refractory valve metal-based anodes with improved volumetric efficiency and D.C. leakage stability. The anodes are particularly suitable for use in high voltage electrolytic capacitors. More specifically, the present invention is related to an improved method of deoxidizing valve metal, particularly niobium and tantalum, anodes and improved capacitors formed with the improved anode.
Manufacturing high voltage electrolytic capacitors requires the use of high formation voltages whereby thick dielectric films are grown. Thick dielectrics are required if the capacitor is to be capable of withstanding high application voltages without degradation and electrical breakdown. It is known, for both Ta and Nb capacitors that electric charge (CV) is not linear with voltage and decreases with increased formation voltage. Capacitance is defined by the C=kA/t wherein t=aVf. Since kA and a are constants for a given anode the equation C=D/Vf is derived wherein D is a constant. Therefore one would expect CV to be flat with formation voltage. Therefore, to achieve high application voltages large anodes are required which limits volumetric efficiency for high capacitance capacitors.
While not limited to any theory, it is thought in the art that limited volumetric efficiency is caused by anodic oxide film growing through the necks formed between powder particles during pressing thereby clogging the pores in sintered anodes. The result is a reduction in surface area of the anodes which causes CV to be non linear in a phenomenon which is referred to in the art as rolling down. Increasing formation voltage in electrolytic capacitors is also believed to be limited by the precipitation of crystalline phases in the amorphous matrix of the anodic oxide dielectric film. The crystalline phases are thought to inhibit formation of a thick amorphous insulating film on the anode surface and they are also thought to provoke high and unstable D.C. leakage. Crystalline phases are typically associated with impurities such as transition metals, carbon and bulk oxygen. The major source of bulk oxygen is native oxide, which dissolves in the bulk of the particles during sintering of the anodes.
U.S. Pat. Nos. 5,825,611; 6,410,083 and 6,554,884 are representative of attempts to address the crystalline oxide problem wherein the Ta or Nb anodes are treated with nitrogen to reduce the affinity of Ta(Nb) for oxygen while limiting nitride precipitation.
U.S. Pat. No. 4,537,641 describes the reduction of bulk oxygen content in Ta(Nb) anodes by adding a reducing agent, such as Mg, to sintered anodes and heating the anodes above the melting point of the reducing agent but below the temperature conventionally used for sintering of valve-metal anodes. During the heating, vaporized reducing agent deposits on the anode surface and reacts with oxygen in Ta(Nb) thereby creating a cover oxide layer of MgO. The oxide coating is thought to avoid the immediate reoxidation of the anode in air. The source of Mg can be Mg powder or Mg chunks placed in the crucibles with the Ta(Nb) anodes. Typical temperature range for deoxidizing is 900° C.-1100° C., depending on powder CV. After the Ta(Nb) anodes are removed from the deoxidizing furnace, the cover oxide layer is chemically leached from the anode surface with material such as a diluted solution of sulfuric acid and hydrogen peroxide.
An alternative process, described in U.S. Pat. No. 6,447,570, is based on a deoxidizing and sintering combination. In this process, referred to in the art as Y-sintering, Ta(Nb) powder is first pressed into a pellet and Mg is added to the pellets. The pellets and Mg are placed in crucibles in a vacuum oven and heat treated under inert atmosphere to generate Mg vapor which forms MgO thereby deoxygenating the Ta(Nb). The sample is then sintered in vacuum, or inert gas, without the anode being exposed to air. When oxygen, which is sintering retardant, is removed from the Ta(Nb) by deoxidizing, the Ta(Nb) particles can be sintered at lower temperatures vs. the temperature conventionally used for sintering of valve-metal anodes. This process provides improved morphology of the sintered anodes with thicker necks between the powder particles and more open pores between the particles. The improvement is believed to be due to surface diffusion of the Ta atoms as an alternative to the bulk diffusion of Ta atoms which dominants at conventional sintering temperatures. During cooling, after sintering, the pellets are treated with nitrogen to reduce the Ta(Nb) affinity for oxygen. After exposure to air the anodes are leached to remove the MgO cover layer. Improved morphology and low oxygen in the Ta(Nb) anodes results in improved volumetric efficiency of the finished Ta(Nb) electrolytic capacitors.
The disadvantage of Y-sintering is that it can not be used for sintering of coarse Ta(Nb) powders such as those used to manufacture high voltage electrolytic capacitors. This is because coarse Ta(Nb) powders with large primary particles require higher sintering temperatures vs. temperatures required for sintering of the higher CV powders with small primary particles. At high sintering temperatures the MgO layer formed on the Ta surface during deoxidizing interacts with Ta(Nb) which contaminates the Ta(Nb) surface thereby inhibiting the formation of a thick insulating film on the anode surface and provoking high and unstable D.C. leakage. Another disadvantage of Y-sintering is that it can not be used efficiently due to the complexity and inefficiency of the equipment needed for its practical realization. During deoxidizing, Mg vapor spreads through the reaction chamber and condenses on all cold parts, including electrical insulation of the heaters. During consequent sintering in vacuum or in inert gas, Mg shunts can cause shortage of the power and control circuits. Long and difficult cleaning of residual Mg must be performed after each run of the furnace.
An alternative process that addresses some of these problems is described in U.S. Pat. No. 7,731,893. According to this process, Ta(Nb) anodes are deoxidized in a deoxidizing furnace using Mg vapor thereby creating a cover layer of MgO and cleaning Ta(Nb) bulk from oxygen. This cover layer prevents the formation of a native oxide on the Ta(Nb) surface when the deoxidized anodes are subsequently exposed to air. Immediately or at a later date, the MgO coated anodes are placed in a separate vacuum oven and sintered. The deoxidized anode, with a MgO coating, can be sintered at lower temperatures than the temperature conventionally used for sintering of valve-metal powder. Sintering at the lower temperature results in an improved morphology and lower oxygen content in the sintered anodes. After cool-down, the MgO cover is removed by a diluted water solution of sulfuric acid and hydrogen peroxide. This sintering process provides essentially the same increased volumetric efficiency as that obtained with Y-sintering; however, it doesn't require any special equipment and maintenance operations and, thereby, is highly productive.
The disadvantages of sintering with a MgO coating in place is the same as the disadvantages of Y-sintering. Neither process can be used for sintering of coarse Ta(Nb) powders because MgO layers formed on Ta(Nb) during deoxidizing interacts with Ta(Nb) at high sintering temperature thereby contaminating the Ta(Nb) surface, and inhibiting high voltage formation which provokes high and unstable D.C. leakage.
There has been an ongoing desire for a method of forming sintered tantalum and niobium anodes which have high volumetric efficiency yet which can be manufactured with minimal effort.